mfd: ac100: Add driver for X-Powers AC100 audio codec / RTC combo IC

The AC100 is a multifunction device with an audio codec subsystem and
an RTC subsystem. These two subsystems share a common register space
and host interface.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>